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In my lab we use a number of materials which are hygroscopic and must therefore be stored in a dry environment. For that purpose we have numerous dry cabinets very similar to the one shown below (Amazon link here).

These cabinets run on electricity, do not require desiccant, and do not have a water outlet such as one finds with home dehumidifiers, yet they are able to maintain low relative humidity (RH) values of < 40%. The temperature inside the cabinet is also the same as (or very close to) the temperature outside of the cabinet, and there are no other connections aside from the electrical plug.

  • How do these cabinets maintain such low humidity values?
  • Assuming they are removing water from the air, where does the water go?

enter image description here

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    $\begingroup$ I'll leave a full answer for later or for another user, but read the text on the last image on that Amazon link. $\endgroup$ – hazzey Aug 13 '15 at 16:42
  • $\begingroup$ @hazzey Thanks, I didn't see that. I wonder how the evaporation of the moisture is forced outside of the cabinet instead of just re-humidifying the interior. Perhaps the moisture is just held as frost indefinitely. $\endgroup$ – Chris Mueller Aug 13 '15 at 17:04
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Not to jump the gun from @hazzey, but one of the product description images describes how the cabinet works:

Cooling

The English in the images is a little lacking (probably a translation from a Chinese manual or something), but it seems like the cabinet condenses the moisture inside, then freezes it on a collection board. Once the space is dehumidified, it appears that the water melts and is wicked to the outside of the enclosure, where it then evaporates into the ambient air.

That is, assuming my interpretation of the image is correct. I'm pretty certain that by the collecting board refreezes to water they mean the ice on the collecting board remelts.

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As I commented and @grfrazee answered, the particular dry cabinet claims to freeze the moisture. I think that there is a general consideration that is being overlooked.

In the question, you compared the dry cabinet to a room dehumidifier, but a better comparison is probably a refrigerator.

A room dehumidifier need to pass a large volume of air through it and take out a lot of water. This is the reason for the bucket or drain hose.

A refrigerator (or dry cabinet) has a much smaller volume of air to work with. Just like a refrigerator doesn't have a drain (it does, but I'll get to that), a dry cabinet won't need a drain. A refrigerator (and I'm assuming that a dry cabinet is just a warmer version) has a small drain pan in its base. The condensation drips onto this and then evaporates normally. The amount of condensation is so low that it never over flows.

Obviously the plumbing and controls will be slightly different, but the relative volume of treated air is similar.

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I believe warmer air has more capacity to hold vapour so instead of condensing onto the items inside the cabinet, the moisture stays in the air. I have 2 cabinets and neither have any outlet so the water vapour can't go anywhere. Also they are relatively cheap to buy as have a simple design.

They seem pretty effective though as mine run at the value I set (45% rh) with no noise and consume around 10 Watts.

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  • $\begingroup$ The moisture holding capacity of air does not increase that much when raised by a "couple of degrees". For instance, 100 F air with 100% humidity would need to be raised to 120 F to get down to 50% humidity. I think @grfrazee's explanation is correct. The water is frozen out of the air onto a cold plate (TE cooler). Then, when the plate is warmed up, the water runs off of the plate into an area where it can evaporate externally. This is similar to cryo vacuum pumps. $\endgroup$ – Chris Mueller Jan 21 at 19:37
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I believe the operation of a dry cabinet is much simpler than that. It just raises the internal temperature to a couple of degrees above ambient which increases the capacity of the air inside to hold vapour. This prevents water from condensing onto the contents.

No moisture is moved to the outside so not much going on really.

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    $\begingroup$ This equipment needs to maintain a low humidity level in the air inside of it, so increasing the water vapor content is the opposite of the desired effect. $\endgroup$ – J. Ari Jan 10 at 14:48

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